This invention relates to idle speed control for a direct fuel injected crankcase scavenged, two-stroke engine, and more particularly to a method and apparatus for stabilizing the rotational speed of such an engine at idle by controlling the time at which fuel is injected during the rotational cycle of the engine.
Customarily, it is desirable to minimize the idle speed of an internal combustion engine to improve fuel economy. However, when an engine is operated near its low speed stall limit, sudden engine loading or changing environmental conditions can decrease engine speed to the point where the generated torque is insufficient to overcome the engine load. As a result the engine stalls. To overcome this difficulty, a control system is generally employed to stabilize engine idle speed at a relatively low value, while insuring that engine stalling does not occur.
One method of controlling the idle speed of a conventional four-stroke engine consists of regulating the quantity of intake air delivered to the engine. This is generally accomplished by adjusting the intake air flow by means of a valve arranged in a passage which bypasses the engine air throttle valve. Unfortunately, a change in the quantity of intake air at idle has a very minimal effect on the output torque produced by a crankcase scavenged, two-stroke engine. Consequently, this method of idle control cannot be used effectively for this type of two-stroke engine.
The idle speed of a four-stroke engine can also be controlled by regulating the quantity of fuel per cylinder delivered to the engine. Combustion of a lean fuel mixture produces less engine output torque, and as a result, engine speed decreases. Again, due to the different operating characteristics of the crankcase scavenged, two-stroke engine, this method of idle control is not desirable. For the two-stroke, leaning out the air-fuel mixture at idle produces increased hydrocarbon emissions, cylinder misfiring, and rough engine idling.
Consequently, an alternative method of idle control is need for the crankcase scavenged, two-stroke engine to minimize engine idle speed for improved fuel economy, while stabilizing rotational speed to prevent engine stalling.